The present invention relates to an electronic still camera, and more particularly to an electronic still camera having an automatic exposure function and a method of determining an exposure time thereof.
Recently, electronic still cameras capable of electronically taking a picture without use of film have been widely used. Such electronic still cameras form a pictorial image by a solid image sensor generally provided with a plurality of photosensitive pixels comprising photoelectronic devices and transfer means for transferring charges produced in the photosensitive pixels. Electronic still cameras ordinarily include an exposure meter incorporated therein and have an automatic exposure function to determine a shutter time on the basis of a measured result of the exposure meter.
For example, in U.S. patent application No. 031,460 (filed on Mar. 30, 1987) by the same applicant as the applicant of this application, an automatic exposure function capable of setting a suitable shutter time following changes in brightness of a subject is disclosed.
However, such a conventional electronic still camera having an automatic exposure function encounters the problems that it is expensive and that errors in setting the shutter speed occur. Namely, since the exposure meter is incorporated in the electronic still camera, a light quantity sensor dedicated to this exposure meter is required, with the result that peripheral components of this light quantity sensor becomes costly. In addition, since measurement of light quantity is conducted at a position somewhat different from an actual image pick-up surface on the solid state image sensor, there is produced a difference between an optimum exposure condition determined on the basis of a measured value of the exposure meter and an actual optimum exposure condition. Especially, since electronic still cameras have a dynamic range narrower than that of ordinary cameras using a silver salt film, if an error in setting the shutter speed occurs, this would greately affect the quality of picture.